陆战

Land combat in Imperator occurs when at least two armies of states currently at war with each other are in the same city. The army that entered the city first is considered the defender, unless the other side has control over fortifications in the province.

On each side, more than 1 army can partake in combat. The simplest way to achieve this is by attaching armies to a leading army. Armies may be attached to allied armies or to the state's own.

Combat

Battle chess board

In Imperator: Rome, the battle chess board is divided into squares, grouped into 2 rows with 30 squares each. Each side has a single row. One cohort fills one square.

Rows have a main front on the middle and flanks on the sides. On the initial deployment the main front is filled with expensive units such as War Elephants, Heavy Cavalry and Heavy Infantry.

Flanks are deployed with mobile units such as Horse Archers, Camel Cavalry and Light Cavalry. These units have a high maneuver which allows them to target units in the center after defeating their opponent.

When units get eliminated they get reinforced by cheap units such as Chariots, Archers and Light Infantry.

How the player can affect deployment

Besides deciding the composition of the army, the player can additionally affect the deployment of troops on the Battle chess board in the following ways:

Primary unit: Gives this unit type the highest priority when deploying the main front.

Secondary unit: Gives this unit type the highest priority when reinforcing the main front.

The default selected choices will depend on the Military Traditions but they can be changed at no cost at any time.

Battle

Each day, each cohort in the first battle row of each side will attack another within its range. A unit type's range in squares is equal to its maneuver value.

The opposing cohorts inflict strength-damage and morale-damage.

Casualties and morale depletion are applied at the end of each day to each cohort taking part in the battle. The Attacker-role and Defender-role are calculated the the same way, but may be subject to some modifiers.

Each cohort has separate base values for strength-damage and morale-damage. These are subject to a random dice roll every 5 days and by other modifiers.

At the end of each day units with less than 0.25 Morale disengage until the end of the battle. On the next day, other units or reserves fill in the ranks to take their place. The day after the last available unit from either side disengages, combat ends and the defeated army is forced into a shattered retreat.
Unlike some other games (e.g. CK2), there is no pursuit phase where the victors run down and inflict heavy casualties upon the broken forces.

Leader modifiers

Terrain modifiers

The attacker receives one or more penalties depending on the city's terrain and map features.
The defender never suffers terrain penalties.
The value can only be zero or less, it cannot be positive.

Hills, Marsh and Forests give -1 to the attacker

Mountains give -2 to the attacker

The following penalties stack on top of any terrain effect:

Crossing a river -1

Attempting a naval landing -2

Crossing a strait -2

Thus the maximum possible penalty is -4 (E.G. a naval landing into mountains).

Discipline

Discipline is a generalized summary of various modifiers to damage done or received in battle. It exists as an overview to aid players.
Discipline itself is a value that serves solely as a modifier, and then other modifiers may adjust the discipline value further. It is calculated on a state level per unit type. When pressing the military button in the top bar, it is possible to see the current modifiers for all unit types.

In a strange exception to this rule, combat tooltip seems to calculate +10% improved discipline from Personal Loyalty to a general twice: both multiplicative and additive. E.g. with +15% country Light Cavalry discipline, a loyal cohort shows +26.5%. The difference of 1.5% is small enough that it's difficult to tell if the tooltip is wrong, or if it's actually calculated this way.

Experience

Every unit accumulates experience when a battle ends. Experience gives ~0.3 damage reduction per 1% of experience. So a unit with 100% experience will receive ~30% less damage. Recovering manpower decays unit's experience with 50% efficiency.

The combat tactic can be changed at any time prior to a battle at no cost.

The effect of any specific tactic is not overwhelmingly powerful. See ##Tips:_Largest_impact to compare the magnitude of other combat modifiers.

General tactics

These five tactics are available to all states.

Tactic

Unit effectiveness

Against other tactics

Casualties

Description

Bottleneck

Archers: 50%

Heavy Cavalry: 80%

Heavy Infantry: 100%

Light Infantry: 25%

Shock Action: +20%

Hit-and-Run: +20%

Skirmishing: −10%

Cavalry Skirmish: −10%

Against a massed charge, nothing performs better than a solid defensive line. However, if the enemy is clever enough to pick us off one by one, we may encounter problems.

Deception

Camel Cavalry: 100%

Chariots: 100%

Horse Archers: 150%

Light Cavalry: 150%

Skirmishing: +20%

Triplex Acies: +20%

Envelopment: −10%

Hit-and-Run: −10%

A staggered assault can wear down an enemy's resolve faster than one might imagine, and allows us to respond to mobile threats with great ease. The greatest weakness of this tactic stems from our vulnerability to skirmishing behavior.

Envelopment

Camel Cavalry: 100%

Chariots: 50%

Heavy Cavalry: 50%

Horse Archers: 50%

Light Cavalry: 100%

Deception: +20%

Phalanx: +20%

Shock Action: −10%

Padma Vyuha: −10%

Drawing forth an enemy counterattack, and then plunging into the side of their exposed formation can cause massive losses. Against an enemy who can quickly martial their men to multiple fronts however, increases the risk of this maneuver.

Shock Action

Heavy Cavalry: 100%

Heavy Infantry: 100%

War Elephants: 200%

Envelopment: +20%

Padma Vyuha: +20%

Bottleneck: −10%

Phalanx: −10%

+10%

Sometimes, caution must be thrown to the wind - few foes can stand against a massed charge, though we must be wary of those that can field a staunch defense.

Ordering cavalry to harass and skirmish, rather than remain in formation, can often be used as a tool to deny an entire flank to hostile troops.

Requires Greek war tradition "The Companion Cavalry", North African war tradition "Wild Charge", or Persian war tradition "Cavalry Skirmish".

Hit-and-Run

Archers: 50%

Camel Cavalry: 50%

Chariots: 50%

Horse Archers: 100%

Light Cavalry: 100%

Light Infantry: 100%

Deception: +25%

Triplex Acies: +25%

Bottleneck: −10%

Padma Vyuha: −10%

−10%

In the face of an overwhelming enemy an asymmetric approach can often be more successful than a head-on one. Ambushes, raids, and hit-and-run style tactics were common in ancient warfare, especially in Gaul, Germania, and Iberia

The Phalanx originated as a highly defensive method of formation fighting, used primarily by Greek city-states. It was further developed by the Macedonian military, who built their armies around a heavily armored Phalanx formation.

Requires Greek war traditions, or Levantine and Arabian war tradition "Greek Warfare".

Triplex Acies

Heavy Infantry: 100%

Light Cavalry: 60%

Light Infantry: 100%

Skirmishing: +25%

Phalanx: +25%

Deception: −10%

Hit-and-Run: −10%

Like the Hellenistic Phalanx the Roman tactic formation known as the Triplex Acies, or triple lines, is inspired by the Phalanx of the Greek City states. Where the Macedonian or Hellenistic Phalanx has gone for cohesion the Roman Formation instead emphasized flexibility.

Damage formula

The damage formula is mostly explained in-game on the damage tooltip. The tooltip is split to 5 sections.

Unit information

Unit type: The tooltip uses the common name of the typ when the modifier is based on the unit's type.

Unit name: The tooltip uses the unit's name when the modifier is based on the unit's attributes.

Unit strength: Multiplier for all damage dealt.

Unit morale: Multiplier for morale damage dealt.

Inflicted losses

This sections shows losses that this unit will cause to the target unit in the next round. The text is misleading indicating these losses were caused this round. However the values use the current strength and morale of the unit, not values in the previous round.

Base damage

Dice roll: Random value between 1 and 6 is rolled every 5 days for each side.

Terrain & Leader modifiers: Added to the dice roll. Terrain modifier causes up to -4 penalty to attacker. General causes up to 7 bonus to the side with a better general (skill difference / 2). (#Terrain_modifiers, #Leader_modifiers)

Tactic vs Tactic: Selected tactic may cause increased or reduced damage against opponent's tactics. Amount of the increased damage depends on unit types in the battlefield. From -10% to +25%. (#Combat_Tactics)

Attacker Offense vs Defender Defense: Units may cause increased damage or receive reduced damage. Main sources are military traditions and trade. Usually from -15% to +30%.

One notable difference from EU4 is that current, not maximum morale, is used to determine morale-damage; so worn down units inflict much less morale-damage on the enemy. Also units that are flanking or in reserve do not suffer any morale hits.

On average you can expect to roll (1+6)/2 = 3.5 and with equal generals and no terrain penalties have a base-damage of 0.8 + 0.2*3.5 = 0.15. In the absence of other modifiers this corresponds to 1000-man cohort with 3.0 morale killing 0.15*1000*0.2 = 30 men and inflicting 0.15*1*(3/2)*1.5 = 0.33 morale damage per day.

Each pip modifies base-damage by 13% of this expected average.
The worst possible base-damage is 0.02, having -4 from terrain and rolling a 1 with an equal or worse general. The best possible is 0.34, rolling a 6 with +7 from general (14-skill general vs no general) and no terrain penalty.

Deployment and reinforcement

All units are priorized based on unit type and location in the army screen. Unit type has the biggest impact while the location is used to resolve ties.

Default priorities for unit types are:

Main front group (units with less than 3 maneuever, ordered by build cost): War Elephants, Heavy Cavalry, Heavy Infantry, Chariots, Archers/Light Infantry